Arc welding apparatus, constant voltage characteristic welding power source, and method for performing arc welding

ABSTRACT

An arc welding apparatus includes a lifting motor. A speed adjustment circuit controls the lifting motor such that the rising speed of the welder decreases if a current of the welding power source is smaller than a set value, or increases if the value of the current is larger than the set value. A voltage adjustment circuit controls the welding power source such that the voltage increases if the number of times that the voltage falls below a determination voltage is larger than a set number of times or if periods for which the voltage remains below the determination voltage are longer than a set time, or decreases if the number of times that the voltage falls below a determination voltage is smaller than a set number of times or if periods for which the voltage remains below the determination voltage is shorter than a set time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an arc welding apparatus, a constantvoltage characteristic welding power source, and a method for performingarc welding.

2. Description of the Related Art

A type of arc welding specialized in vertical-up welding in which highefficiency is obtained by filling a surface of a welding groove througha one-pass operation is called “electrogas arc welding” including anapparatus, and has been widely put into practice (for example, refer toJapanese Unexamined Patent Application Publication No. 59-130689,Japanese Patent No. 3596723, and Japanese Unexamined Patent ApplicationPublication No. 2004-167600).

In the basic mechanism of the apparatus, a welding torch, a wire feedmotor, a sliding copper plate including a gas supply port and a coolingtube, and a dedicated device (welder) including a lifting motor aremounted on a rail extending on a steel plate in a longitudinal directionof the groove, and welding is performed in the groove while lifting thewelder in accordance with the rise of a surface of a molten pool.

For such welding, some methods for linking the rise of the surface ofthe molten pool with the rise of the dedicated device have beenproposed.

In a first method, a link mechanism is not used. That is, this is acompletely manual method in which the speed of the lifting motor ismanually adjusted while constantly observing the surface of the moltenpool. The configuration of the apparatus is the simplest, but constantobservation is required. The workload is too heavy and the quality ofwelding obtained is too unstable to perform welding of a welding lengthof several meters to tens of meters.

Therefore, in order to reduce the workload and stabilize the quality ofwelding, an automatic rising mechanism has been proposed.

That is, a second method is a control method in which an optical sensoris mounted on the apparatus and a linkage with the rising speed of thesurface of the molten pool is established by utilizing a mechanism inwhich the intensity of arc light changes in accordance with the rise ofthe surface of the molten pool.

However, this method is not stable since the arc is not always stableand affects the intensity of light and fumes (smoke) generated duringthe welding blocks the light irregularly.

Therefore, as a third method, a method in which feedback to the risingspeed is performed using values of current may be used. This iscurrently the most popular method.

In this method, a constant voltage characteristic welding power sourceis used as a welding power source. When a wire feed rate has beendetermined, the constant voltage characteristic welding power sourceoutputs a current large enough to melt a wire. The welding wire ismelted by total energy of arc heat proportional to the product of thecurrent and the potential difference of the arc (arc voltage), theelectrical resistivity of the welding wire itself, a welding distance,and the product of the square of the current, and, as a result, thecurrent and the welding distance are balanced. Here, because the weldingdistance becomes shorter and resistance heat decreases when the surfaceof the molten pool rises, the welding power source increases the outputcurrent thereof in order to supplement insufficient melting energy.Therefore, by instantly reading this increase in the output voltage andincreasing the rising speed of the welder, the welding distance becomeslonger again and the resistance heat increases. Because the meltingenergy becomes excessive, the output current is reduced, therebysuppressing the rising speed. By repeating this procedure continuously,the rise of the surface of the molten pool and the rise of the welderare linked to each other, and accordingly monitoring is no longerrequired.

That is, currently, only the feedback control of the current and therising speed has been put into practice, and an arc length, which is anessential factor in the quality of welding, is not controlled at allexcept for the automatic control characteristic of the constant voltagepower source. The automatic control characteristic of the constantvoltage power source may be referred to as a function of maintaining aset arc length regardless of whether or not the set arc length isappropriate.

Voltage as a parameter in welding may be regarded as equivalent to thearc length, but in vertical-up welding of a large welding length, thearc length and the voltage need to be considered more strictly than indownhand welding or horizontal welding. This is because of differencesin an arc force direction and a penetration direction.

FIGS. 11A and 11B are schematic diagrams illustrating a weldingdirection, the arc force direction, and the penetration direction ineach welding attitude. FIG. 11A illustrates downhand welding, and FIG.11B illustrates vertical-up welding. Hollow arrows indicate the weldingdirections, broken-line arrows indicate the arc force directions, andthick-solid-line arrows indicate the penetration directions.

As illustrated in FIG. 11A, in downhand welding, the arc force directionand the penetration direction in the groove are parallel to each other.Therefore, deep penetration is structurally easy to obtain, and fewfailures occur in penetration. Although it is known that theconcentration of an arc depends on the arc length and accordingly thearc length affects the penetration, the degree of effect is smallcompared to that in vertical-up welding, which will be describedhereinafter.

On the other hand, as illustrated in FIG. 11B, in vertical-up welding,the arc force direction and the penetration direction in a widthdirection of the groove are perpendicular to each other. That is, thearc force does not directly affect the penetration. In vertical-upwelding, penetration in the width direction is obtained due toconvection generated in the molten pool immediately below the arc.Therefore, the intensity of the convection determines the depth ofpenetration, and the penetration is shallow relative to input heatenergy. Moreover, because the convection in the molten pool is easilyaffected by the distribution of arc force, small variations in the arclength cause failures in the penetration. That is, in vertical-upwelding, appropriate control of the arc length is significantlyimportant in terms of securing the quality of welding.

In the downhand welding illustrated in FIG. 11A, the penetration isgenerally deep when the arc length is small, whereas in the verticalwelding illustrated in FIG. 11B, the penetration is deep when the arclength is large. These opposite characteristics derive from theabove-described difference between their respective mechanisms.

Currently, however, the arc length and the voltage are not controlled atall as described above. Since the arc length is generally consideredequivalent to the voltage, a value of voltage corresponding to a certainvalue of current is managed as a model condition, but a problem arisingin this case is the reliability of the absolute value of voltage.

FIGS. 12A and 12B are diagrams illustrating relationships between arcvoltage and a voltage loss in cables.

As illustrated in FIGS. 12A and 12B, power source output voltageV_(power) output from a welding power source includes not only arcvoltage V_(arc), which is the potential difference of an arc, but also avoltage loss ΣV_(cable) in secondary cables connecting the welding powersource and a welding torch and the welding power source and a base metalor connecting portions (V_(power)=V_(arc)+ΣV_(cable)).

That is, in the secondary cables and the connecting portions, part ofpower is converted into a heat loss due to the voltage loss(τV_(cable)). The voltage loss is negligible when the secondary cablesare short, but in ships, bridge piers, tanks, and the like, which aretargets of the present invention, the voltage loss cannot be neglectedbecause welding of a large welding length is performed by lifting awelder mounted with cables having a length of tens of meters while usinga welding power source fixed on the ground.

For example, even when 37 V has been set as the power source outputvoltage V_(power) that serves as a desirable welding condition, the arcvoltage V_(arc) varies depending on the lengths of the secondary cables.When the secondary cables are long and the voltage loss is large asillustrated in FIG. 12A, the arc voltage V_(arc), which is thedifference between the power source output voltage V_(power) and thevoltage loss, becomes low. As a result, convection in a molten poolbecomes weak and penetration becomes shallow. On the other hand, whenthe secondary cables are short and the voltage loss is small asillustrated in FIG. 12B, the arc voltage V_(arc), which is thedifference between the power source output voltage V_(power) and thevoltage loss, becomes high. As a result, convection in a molten poolbecomes strong and penetration becomes deep. Thus, it cannot be saidthat the penetration is controlled. Furthermore, although thepenetration can be secured when the arc voltage is high, the mechanicalproperties of a weld metal may deteriorate when the arc voltage isexcessive because the components of the weld metal become inappropriatedue to significant oxidation reaction in the arc and inevitable mixingof the atmosphere in the arc.

In addition, in the case of downhand welding, even if the lengths of thesecondary cables are not taken into consideration, failures in thepenetration may be substantially prevented insofar as the arc is incontact with the surface of the groove, which may be adjusted by anoperator during the welding. In the case of vertical welding, however,since the arc does not come into contact with the surface of the groove,it is difficult for the operator to determine whether or not appropriatepenetration is being obtained. That is, even when current conditions orvoltage conditions are inappropriate from the beginning, it is difficultfor the operator to tell that. As a result, in the worst case, failuresin the penetration and failures in the properties of the weld metaloccur along the entirety of the welding length.

As described above, currently, only the control of the rising speed hasbeen put into practice in a vertical welding apparatus, that is, onlythe shape of a weld portion is controlled, and the arc length and thevoltage, which are two of other important welding conditions, are notcontrolled at all such that the arc length and the voltage becomeappropriate. Therefore, the stability of the penetration quality and themechanical properties of the weld metal is substantially not controlledat all and fully dependent on the empirical intuition of the operator.For this reason, it has been desired to improve the apparatus to achievefurther automation, elimination of the need for monitoring, andstabilization of quality.

In the technique disclosed in Japanese Unexamined Patent ApplicationPublication No. 59-130689, changes in the arc length and the voltage,which is a quantitative value of the arc length, according to the riseof the surface of the molten pool are read and used to control a liftingmotor, but this is the same as the above-described feedback controlbetween changes in current and the lifting motor, and whether or not theabsolute value of the arc length is appropriate is not controlled.

In addition, the techniques disclosed in Japanese Patent No. 3596723 andJapanese Unexamined Patent Application Publication No. 2004-167600, too,do not propose a method for controlling whether or not the absolutevalue of the arc length is appropriate.

An object of the present invention is to increase the possibility thatthe arc length is continuously maintained constant when vertical-upwelding is performed by generating an arc in a groove between steelplates to be welded and forming a molten pool.

SUMMARY OF THE INVENTION

In view of such an object, an aspect of the present invention providesan arc welding apparatus that performs vertical-up welding by generatingan arc in a groove between steel plates to be welded and forming amolten pool. The arc welding apparatus includes welding means forperforming arc welding by generating the arc from a welding wire in thegroove between the steel plates to be welded in a substantiallyvertically downward direction and forming the molten pool, lifting meansfor lifting the welding means in a substantially vertically upwarddirection relative to the steel plates to be welded, a welding powersource that feeds current to the welding wire to generate the arc, speedcontrol means for monitoring output current output from the weldingpower source and, if a value of the output current is smaller than avalue of current set in advance, controlling the lifting means such thatrising speed of the welding means decreases or, if the value of theoutput current is larger than the set value of current, controlling thelifting means such that the rising speed of the welding means increases,and voltage control means for monitoring output voltage output from thewelding power source during the welding, detecting information regardingthe number of times that or periods for which a value of the outputvoltage falls below a determination voltage, which is set in advance asa determination threshold, and, if the information regarding the numberof times or the periods exceeds the set threshold, controlling thewelding power source such that the value of the output voltage increasesor, if the information regarding the number of times or the periods isbelow the set threshold, controlling the welding power source such thatthe value of the output voltage decreases.

Here, the information regarding the number of times that or the periodsfor which the value of the output voltage falls below the determinationvoltage may be the number of times in unit time that the value of theoutput voltage falls below the determination voltage. The set thresholdmay be the number of times set in advance as the number of times in theunit time. When the determination voltage is 15 V, the set number oftimes may be any number of times within a range from 3 times per secondto 60 times per second.

Alternatively, the information regarding the number of times that or theperiods for which the value of the output voltage falls below thedetermination voltage may be a period obtained on the basis of apredetermined number of periods for which the value of the outputvoltage remains below the determination voltage. The set threshold maybe time set in advance. In addition, when the determination voltage is15 V, the set time may be any time within a range from 0.1 ms to 1.0 ms.

Furthermore, the speed control means may control the lifting means suchthat the rising speed of the welding means becomes lower than or equalto 180 mm/min.

In addition, another aspect of the present invention is an arc weldingapparatus that performs vertical-up welding by generating an arc in agroove between steel plates to be welded and forming a molten pool. Thearc welding apparatus includes a backing material mounted across a rootgap provided in a back of the groove between the steel plates to bewelded, welding means for performing arc welding by generating the arcfrom a welding wire in the groove between the steel plates to be weldedin a substantially vertically downward direction and forming the moltenpool, the welding means including a welding torch that is arranged infront of the groove between the steel plates to be welded and thatsupplies a welding wire into the groove, a weaving mechanism thatoscillates the welding torch in a width direction of the groove, and asliding copper plate that relatively slides over front surfaces of thesteel plates to be welded in a substantially vertically upwarddirection, lifting means for lifting the welding means in thesubstantially vertically upward direction relative to the steel platesto be welded, a welding power source that feeds current to the weldingwire to generate the arc, speed control means for monitoring outputcurrent output from the welding power source and, if a value of theoutput current is smaller than a value of current set in advance,controlling the lifting means such that rising speed of the weldingmeans decreases or, if the value of the output current is larger thanthe set value of current, controlling the lifting means such that therising speed of the welding means increases, and voltage control meansfor monitoring output voltage output from the welding power sourceduring the welding, detecting information regarding the number of timesthat or periods for which a value of the output voltage falls below adetermination voltage, which is set in advance as a determinationthreshold, and, if the information regarding the number of times or theperiods exceeds the set threshold, controlling the welding power sourcesuch that the value of the output voltage increases or, if theinformation regarding the number of times or the periods is below theset threshold, controlling the welding power source such that the valueof the output voltage decreases.

Furthermore, another aspect of the present invention is an arc weldingapparatus that performs vertical-up welding by generating an arc in agroove between steel plates to be welded and forming a molten pool. Thearc welding apparatus includes welding means for performing arc weldingby generating arcs from a plurality of welding wires in the groovebetween the steel plates to be welded in a substantially verticallydownward direction and forming the molten pool, lifting means forlifting the welding means in a substantially vertically upward directionrelative to the steel plates to be welded, a plurality of welding powersources that feed current to the plurality of welding wires to generatethe arcs, speed control means for monitoring output current output fromone of the plurality of welding power sources and, if a value of theoutput current is smaller than a value of current set in advance,controlling the lifting means such that rising speed of the weldingmeans decreases or, if the value of the output current is larger thanthe set value of current, controlling the lifting means such that therising speed of the welding means increases, and a plurality of voltagecontrol means for monitoring output voltages output from the pluralityof welding power sources during the welding, detecting informationregarding the number of times that or periods for which values of theoutput voltages fall below a determination voltage, which is set inadvance as a determination threshold, and, if the information regardingthe number of times or the periods exceeds the set threshold,controlling the welding power sources such that the values of the outputvoltages increase or, if the information regarding the number of timesor the periods is below the set threshold, controlling the welding powersources such that the value of the output voltages decrease.

Furthermore, another aspect of the present invention is a constantvoltage characteristic welding power source used for an arc weldingapparatus that performs vertical-up welding by lifting, in asubstantially vertically upward direction relative to steel plates to bewelded, a welder that performs welding by generating an arc from awelding wire in a groove between the steel plates to be welded in asubstantially vertically downward direction and forming a molten pool.The constant voltage characteristic welding power source includes powersupply means for feeding current to the welding wire to generate thearc, speed control means for monitoring output current output from thepower supply means and, if a value of the output current is smaller thana value of current set in advance, controlling the welder such thatrising speed of the welder decreases or, if the value of the outputcurrent is larger than the set value of current, controlling the weldersuch that the rising speed of the welder increases, and voltage controlmeans for monitoring output voltage output from the power supply meansduring the welding, detecting information regarding the number of timesthat or periods for which a value of the output voltage falls below adetermination voltage, which is set in advance as a determinationthreshold, and, if the information regarding the number of times or theperiods exceeds the set threshold, performing control such that thevalue of the output voltage increases or, if the information regardingthe number of times or the periods is below the set threshold,performing control such that the value of the output voltage decreases.

On the other hand, another aspect of the present invention is a methodfor performing arc welding in which vertical-up welding is performed bylifting, in a substantially vertically upward direction relative tosteel plates to be welded, a welder that performs welding by generatingan arc from a welding wire in a groove between the steel plates to bewelded in a substantially vertically downward direction and forming amolten pool. The method includes the steps of monitoring output currentoutput from a welding power source that feeds current to the weldingwire to generate the arc and, if a value of the output current issmaller than a value of current set in advance, controlling the weldersuch that rising speed of the welder decreases or, if the value of theoutput current is larger than the set value of current, controlling thewelder such that the rising speed of the welder increases, andmonitoring output voltage output from the welding power source duringthe welding, detecting information regarding the number of times that orperiods for which a value of the output voltage falls below adetermination voltage, which is set in advance as a determinationthreshold, and, if the information regarding the number of times or theperiods exceeds the set threshold, performing control such that thevalue of the output voltage increases or, if the information regardingthe number of times or the periods is below the set threshold,performing control such that the value of the output voltage decreases.

According to the present invention, it is possible to increase thepossibility that the arc length is continuously maintained constant whenvertical-up welding is performed by generating an arc in a groovebetween steel plates to be welded and forming a molten pool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a case in which the number of shortcircuits is used as a parameter for controlling an arc length;

FIG. 2 is a diagram illustrating a case in which short circuit periodsare used as the parameter for controlling the arc length;

FIG. 3 is a schematic diagram illustrating the configuration of awelding apparatus according to a first embodiment;

FIG. 4 is a schematic diagram illustrating the configuration of awelding apparatus in the related art as a comparative example of thewelding apparatus according to the first embodiment;

FIG. 5 is a schematic diagram illustrating the configuration of awelding apparatus according to a second embodiment;

FIG. 6 is a schematic diagram illustrating the configuration of awelding apparatus in the related art as a comparative example of thewelding apparatus according to the second embodiment;

FIG. 7 is a flowchart illustrating an example of the operation of aspeed adjustment circuit according to the first embodiment;

FIG. 8 is a flowchart illustrating a first example of the operation of avoltage adjustment circuit according to the first embodiment;

FIG. 9 is a flowchart illustrating a second example of the operation ofthe voltage adjustment circuit according to the first embodiment;

FIGS. 10A to 10D are diagrams illustrating examples of welding to whichthe present invention may be applied;

FIGS. 10E and 10F are diagrams illustrating examples of the welding towhich the present invention may be applied;

FIGS. 11A and 11B are schematic diagrams illustrating a weldingdirection, an arc force direction, and a penetration direction in eachwelding attitude; and

FIGS. 12A and 12B are diagrams illustrating relationships between powersource output voltage, arc voltage, and a voltage loss in cables.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detailhereinafter with reference to the accompanying drawings.

An object of the embodiments is to perform control such that an arclength (arc voltage) constantly remains at an appropriate value in anycase. In order to achieve this object, first, the inventors have studiedabout appropriate welding conditions, and found out that, in vertical-upwelding, short circuits occur at certain intervals under an arc lengthcondition under which appropriate penetration may be secured anddesirable properties of a weld metal may be obtained. A short circuit isa phenomenon in which an arc momentarily disappears and a leading end ofa welding wire comes into contact with a surface of a molten pool.

When the number of short circuits in a certain period of time is large,failures in penetration occur because the arc length is large. On theother hand, when the number of short circuits is small, the propertiesof the weld metal deteriorate because the arc length is small.

The inventors have also found out that the arc length may be maintainedat an appropriate value more accurately by measuring and controlling notthe number of short circuits but periods (short circuit periods) forwhich short circuits are occurring.

By utilizing this phenomenon, a mechanism for sampling, analyzing, anddetermining voltage and issuing an output voltage instruction to awelding power source is invented for a welding apparatus.

With respect to actual control, because a short circuit is a phenomenonthat lasts an extremely short period of time and accordingly it istechnically difficult to observe a timing at which voltage becomes closeto zero due to disappearance of an arc, an analysis is performed whileregarding a time at which the voltage falls below a certain threshold asthe beginning of a short circuit. In the embodiments, the threshold isdefined as a determination voltage V_(short), but the threshold may bean appropriate value at which control of the arc length becomes moststable in accordance with the lengths of secondary cables, weldingconditions, a welding material, shielding gas, and the like. However, noproblem arises insofar as the threshold is set between 10 V and 18 V,and the determination voltage V_(short) is set to 15 V herein.

FIG. 1 is a diagram illustrating a case in which the number of shortcircuits is used as a parameter for controlling the arc length.

In this case, as illustrated in FIG. 1, changes in voltage, that is,voltage falling below the determination voltage V_(short) and exceedingthe determination voltage V_(short) again, are counted as one shortcircuit, and the number of short circuits occurred in a certain periodof time T_(s) is divided by the certain period of time T_(s) in order toobtain N_(short) [times/s]. The certain period of time T_(s) may bearbitrarily set in order to make the control most effective. The certainperiod of time T_(s) is effective when the certain period of time T_(s)is 0.1 to 1 second, but no problem arises when the certain period oftime T_(s) is about 0.5 second. However, when the determination voltageT_(s) is too small, variation in N_(short) becomes large, which can makethe control unstable. On the other hand, when the determination voltageT_(s) is too large, response speed in the control becomes low, which canalso make the control unstable because it is difficult to instantly copewith a sharp change in the arc length.

If the determination voltage T_(s) is 0.5 second in FIG. 1, N_(short) is8 times/s. The calculated N_(short) is compared with a set number oftimes N_(set), and a voltage output instruction according to a result isissued. This operation is sequentially performed as a routine in orderto constantly maintain the arc length at an appropriate value.

FIG. 2 is a diagram illustrating a case in which short circuit periodsare used as the parameter for controlling the arc length.

In this case, as illustrated in FIG. 2, times taken for the voltage toexceed the determination voltage V_(short) again after falling below thedetermination voltage V_(short) are measured as T_(short1), T_(short2),and so on, and an average of a certain number of times is calculated asan average short circuit period Ave_(Tshort) and used for a comparison.More specifically, the average short circuit period Ave_(Tshort) isobtained by dividing a sum ΣT_(short) of the times T_(short1),T_(short2), and so on by the number of short circuits. The number ofshort circuits used to calculate the average value may be arbitrarilydetermined in order to make the control most effective. The number ofshort circuits is effective when the number of short circuits is 3 to20, but no problem arises when the number of short circuits is about 5.However, when the number of short circuits used to calculate the averagevalue is too small, variation in the average short circuit periodAve_(Tshort) becomes large, which can make the control unstable. On theother hand, when the number of short circuits used to calculate theaverage value is too large, the response speed in the control becomeslow, which can also make the control unstable because it is difficult toinstantly cope with a sharp change in the arc length.

In FIG. 2, the calculated average short circuit period Ave_(Tshort) iscompared with a set time T_(set), and a voltage output instructionaccording to a result is issued. This operation is sequentiallyperformed as a routine in order to constantly maintain the arc length atan appropriate value.

Next, a method for realizing such control will be described in detail.Because electrogas arc welding may be typically of a one-electrode typeor a two-electrode type, the former will be described as a firstembodiment and the latter will be described as a second embodiment.

First, a method for realizing control according to the first embodimentwill be described.

FIG. 3 is a schematic diagram illustrating the configuration of awelding apparatus 1 according to the first embodiment.

As illustrated in FIG. 3, the welding apparatus 1 according to the firstembodiment includes a welding robot 10 for performing electrogas arcwelding in which welding is performed by generating an arc from anelectrode and an operation display box 20 for operating the weldingrobot 10 and displaying information regarding the welding robot 10. Inaddition, the welding apparatus 1 includes a welding power source 30that supplies power for the welding and secondary cables 40 throughwhich a current output from the welding power source 30 flows.Furthermore, the welding apparatus 1 includes a speed adjustment circuit50 that adjusts the rising speed of a welder of the welding robot 10 onthe basis of the current flowing through the secondary cables 40 and avoltage adjustment circuit 60 that adjusts output voltage of the weldingpower source 30 on the basis of the voltage output from the weldingpower source 30.

The welding robot 10 includes a welding torch 11 for forming a moltenpool P in a groove G extending in a vertical direction of a base metalB, which is configured by a pair of steel plates, or a direction closeto the vertical direction, a wire feed motor 12 that feeds a weldingwire that functions as the electrode to the welding torch 11, a backingmaterial 13 mounted on a back of the groove G, and a sliding copperplate 14 mounted on a front of the groove G. The welding robot 10 alsoincludes a carriage 15 that is linked to the welder, which includes thewelding torch 11, the wire feed motor 12, and the sliding copper plate14, and that rises in accordance with the rise of the surface of themolten pool P, a lifting motor 16 that lifts the carriage 15 at aspecified speed, and a rail 17 that guides the rise of the carriage 15.

The welding torch 11 includes the welding wire, which generates an arcwhen voltage is applied from the welding power source 30. The weldingtorch 11 may perform weaving in a direction indicated by an arrow Wusing a weaving motor, which is not illustrated.

The wire feed motor 12 feeds the welding wire to the welding torch 11from a wire reel, which is not illustrated and on which the welding wireis wound, in order to feed the welding wire to the groove G. At thistime, a rate F at which the wire feed motor 12 feeds the welding wire isdetermined by converting a set value of current I_(set), which will bedescribed later.

The backing material 13 is a member mounted across a root gap in theback of the groove G, and may be composed of a metal or a nonmetal.Alternatively, the backing material 13 is not provided when the basemetal B does not include a root gap.

The sliding copper plate 14 is a copper plate capable of sliding in alongitudinal direction of the groove G relative to the groove G. A gassupply port 141 is provided in the sliding copper plate 14, andshielding gas supplied from the gas supply port 141 covers the arc inorder to prevent air from entering a welding atmosphere. A cooling tube142 is also provided in the sliding copper plate 14, and water flowingthrough the cooling tube 142 cools the molten pool P through the slidingcopper plate 14 in order to make the molten pool P become a weld metalM.

The carriage 15 is guided by the rail 17 and rises in the longitudinaldirection of the groove G. As a result, the welding torch 11, the wirefeed motor 12, and the sliding copper plate 14 also rise in thelongitudinal direction (a direction indicated by a hollow arrow U inFIG. 3) of the groove G.

The lifting motor 16 lifts the carriage 15 at a speed based on aninstruction from the speed adjustment circuit 50.

The rail 17 is a steel member extending on the base metal B in thelongitudinal direction of the groove G.

The operation display box 20 is an apparatus used to, for example,specify the welding conditions before the welding robot 10 begins tooperate. Here, the welding conditions include the set value of currentI_(set) and the determination voltage V_(short). The welding conditionsalso include the set number of times N_(set), which is a threshold forthe number of short circuits in the certain period of time, and the settime T_(set), which is a threshold for the average short circuit period.Although not illustrated, the operation display box 20 includes adisplay screen configured by a liquid crystal display or the like andinput buttons. Alternatively, the operation display box 20 may be aknown touch panel adopting, for example, a capacitive method in which aposition touched by a finger is electrically detected by detecting achange in the surface charge of the panel on which a low-voltageelectric field has been formed or a resistive method in which a positiontouched by a finger is electrically detected by detecting a change froma nonconductive state to a conductive state at a position of electrodesthat are separated from each other.

The welding power source 30 is a constant voltage characteristic weldingpower source, and, when the rate F at which the wire feed motor 12 feedsthe welding wire has been determined, outputs a current large enough tomelt the welding wire.

Each of the secondary cables 40 are cables connecting the welding powersource 30 and the welding wire included in the welding torch 11 or thebase metal B. The secondary cables 40 connect a positive terminal of thewelding power source 30 to the welding wire and a negative terminal ofthe welding power source 30 to the base metal B.

The speed adjustment circuit 50 includes a current sampling circuit 51that samples the current flowing through the secondary cables 40 and adetermination circuit 52 that determines the current sampled by thecurrent sampling circuit 51 and that issues a lifting speed instructionto the lifting motor 16.

The current sampling circuit 51 monitors a current I_(power) output fromthe welding power source 30 and flowing through the secondary cables 40.

When the current I_(power) is smaller than the set value of currentI_(set), the determination circuit 52 issues an instruction to decreasea rising speed S_(up) of the carriage 15, and when the current I_(power)is equal to the set value of current I_(set), the determination circuit52 issues an instruction to maintain the rising speed S_(up) of thecarriage 15. When the current I_(power) is larger than the set value ofcurrent I_(set), the determination circuit 52 issues an instruction toincrease the rising speed S_(up).

The voltage adjustment circuit 60 includes a voltage sampling circuit 61that samples the voltage output from the welding power source 30, awaveform analysis circuit 62 that analyzes the waveform of the voltagesampled by the voltage sampling circuit 61 and that outputs a result ofthe analysis, and a determination circuit 63 that issues an outputvoltage instruction to the welding power source 30 in accordance withthe result of the analysis obtained by the waveform analysis circuit 62.

The voltage sampling circuit 61 monitors voltage V_(power) output fromthe welding power source 30 during welding.

The waveform analysis circuit 62 counts the number of times N_(short)[times/s] in unit time that the voltage V_(power) falls below thecertain determination voltage V_(short). Alternatively, the waveformanalysis circuit 62 calculates the average short circuit periodAve_(Tshort) Ed of a certain number of periods for which the voltageV_(power) remains below the certain determination voltage V_(short).

When the number of times N_(short) is larger than the set number oftimes N_(set), the determination circuit 63 issues an instruction toincrease the voltage V_(power), and when the number of times N_(short)is equal to the set number of times N_(set), the determination circuit63 issues an instruction to maintain the voltage V_(power). When thenumber of times N_(short) is smaller than the set number of timesN_(set), the determination circuit 63 issues an instruction to decreasethe voltage V_(power). Alternatively, when the average short circuitperiod Ave_(Tshort) of the certain number of periods is longer than theset time T_(set), the determination circuit 63 issues an instruction toincrease the voltage V_(power), and when the average short circuitperiod Ave_(Tshort) is equal to the set time T_(set), the determinationcircuit 63 issues an instruction to maintain the voltage V_(power). Whenthe average short circuit period Ave_(Tshort) is shorter than the settime T_(set), the determination circuit 63 issues an instructiondecrease the voltage V_(power).

FIG. 4 is a schematic diagram illustrating the configuration of awelding apparatus 1 in the related art as a comparative example of thewelding apparatus 1 according to the first embodiment.

The welding apparatus 1 in the related art does not include the voltageadjustment circuit 60, which is included in the welding apparatus 1according to the first embodiment illustrated in FIG. 3, and the weldingconditions thereof specified by the operation display box 20 onlyinclude a set value of voltage V_(set) and do not include thedetermination voltage V_(short), the set number of times N_(set), andthe set time T_(set). Therefore, detailed description of each componentis omitted.

Although the voltage adjustment circuit 60 is configured as a so-calledcontrol box independent of the welding power source 30 in the firstembodiment, the present invention is not limited to this. When thevoltage adjustment circuit 60 is included in the welding power source 30and the welding power source 30 is used as a dedicated power source,portability and connectability improve, which makes the welding powersource 30 more convenient.

Although not illustrated in FIG. 3, a voltage adjustment function(realized by a dial or digital setting) that adjusts the output voltageto the set value of voltage V_(set) and that is invariably used by ageneral welding power source 30 is valid only during arc startup hereinregardless of whether the voltage adjustment circuit 60 is a control boxor included in the welding power source 30. This is because the voltageadjustment function is generally a function of enabling an operator toconsciously adjust the arc length, but since the arc length isautomatically adjusted to an optimal value in the present embodiment, anadjustment operation performed by the operator using his/her ownphysical functions and determinations is not necessary whereas, onlyduring the arc startup, the control according to the present embodimentdoes not function due to its calculation mechanism. That is, the voltageadjustment function according to the present embodiment exists todetermine arc startup voltage. However, because the voltage adjustmentfunction hardly affects the welding quality, the importance of thevoltage adjustment function is significantly lower than that in thewelding apparatus 1 in the related art.

Next, a method for realizing control according to the second embodimentwill be described.

FIG. 5 is a schematic diagram illustrating the configuration of awelding apparatus 1 according to the second embodiment.

As illustrated in FIG. 5, the welding apparatus 1 according to thesecond embodiment includes a welding robot 10 for performing electrogasarc welding in which welding is performed by generating arcs fromelectrodes and an operation display box 20 for operating the weldingrobot 10 and displaying information regarding the welding robot 10. Inaddition, the welding apparatus 1 includes welding power sources 30 aand 30 b that supply power for the welding, a secondary cable 40 athrough which a current output from the welding power source 30 a flows,a secondary cable 40 b through which a current output from the weldingpower source 30 b flows, and a secondary cable 40 c through which thecurrents output from the welding power sources 30 a and 30 b flow.Furthermore, the welding apparatus 1 includes a speed adjustment circuit50 a that adjusts the rising speed of a welder of the welding robot 10on the basis of the current flowing through the secondary cable 40 a, avoltage adjustment circuit 60 a that adjusts output voltage of thewelding power source 30 a on the basis of the voltage output from thewelding power source 30 a, and a voltage adjustment circuit 60 b thatadjusts output voltage of the welding power source 30 b on the basis ofthe voltage output from the welding power source 30 b.

The welding robot 10 includes welding torches 11 a and 11 b for forminga molten pool P in a groove G extending in a vertical direction of abase metal B, which is configured by a pair of steel plates, or adirection close to the vertical direction, wire feed motors 12 a and 12b that feed welding wires that function as the electrodes to the weldingtorches 11 a and 11 b, respectively, a backing material 13 mounted on aback of the groove G, and a sliding copper plate 14 mounted on a frontof the groove G. The welding robot 10 also includes a carriage 15 thatis linked to the welder, which includes the welding torches 11 a and 11b, the wire feed motors 12 a and 12 b, and the sliding copper plate 14,and that rises in accordance with the rise of the surface of the moltenpool P, a lifting motor 16 that lifts the carriage 15 at a specifiedspeed, and a rail 17 that guides the rise of the carriage 15.

The welding torches 11 a and 11 b include the welding wires,respectively, which generate arcs when voltage is applied from thewelding power sources 30 a and 30 b, respectively. The welding torches11 a and 11 b may perform weaving in directions indicated by arrows Wusing weaving motors, which are not illustrated.

The wire feed motors 12 a and 12 b feed the welding wires to the weldingtorches 11 a and 11 b, respectively, from wire reels, which are notillustrated and on which the welding wires are wound, in order to feedthe welding wires to the groove G. At this time, a rate F₁ at which thewire feed motor 12 a feeds the welding wire is determined by convertinga set value of current I_(set1), which will be described later, and arate F₂ at which the wire feed motor 12 b feeds the welding wire isdetermined by converting a set value of current I_(set2), which will bedescribed later.

The backing material 13, the sliding copper plate 14, the carriage 15,the lifting motor 16, and the rail 17 are the same as those describedwith reference to the first embodiment, and accordingly descriptionthereof is omitted.

The operation display box 20 is an apparatus used to, for example,specify the welding conditions before the welding robot 10 begins tooperate. Here, the welding conditions include the set value of currentI_(set1), a determination voltage V_(short1), the set value of currentI_(set2), and a determination voltage V_(short2). The welding conditionsalso include a set number of times N_(set1), which is a threshold forthe number of short circuits in a certain period of time, and a set timeT_(set1), which is a threshold for the average short circuit period.Furthermore, the welding conditions include a set number of timesN_(set2), which is a threshold for the number of short circuits in thecertain period of time, and a set time T_(set2), which is a thresholdfor the average short circuit period. Although not illustrated, theoperation display box 20 includes a display screen configured by aliquid crystal display or the like and input buttons. Alternatively, theoperation display box 20 may be a known touch panel adopting, forexample, a capacitive method in which a position touched by a finger iselectrically detected by detecting a change in the surface charge of thepanel on which a low-voltage electric field has been formed or aresistive method in which a position touched by a finger is electricallydetected by detecting a change from a nonconductive state to aconductive state at a position of electrodes that are separated fromeach other.

The welding power source 30 a is a constant voltage characteristicwelding power source, and, when the rate F₁ at which the wire feed motor12 a feeds the welding wire has been determined, outputs a current largeenough to melt the welding wire. When the rate F₂ at which the wire feedmotor 12 b feeds the welding wire has been determined, the welding powersource 30 b outputs a current large enough to melt the welding wire.

The secondary cable 40 a is a cable connecting the welding power source30 a and the welding wire included in the welding torch 11 a, thesecondary cable 40 b is a cable connecting the welding power source 30 band the welding wire included in the welding torch 11 b, and thesecondary cable 40 c is a cable connecting the welding power sources 30a and 30 b and the base metal B. The secondary cables 40 a and 40 bconnect positive terminals of the welding power sources 30 a and 30 b,respectively, to the welding wires, and the secondary power source 40 cconnects negative terminals of the welding power sources 30 a and 30 bto the base metal B.

The speed adjustment circuit 50 a includes a current sampling circuit 51a that samples the current flowing through the secondary cable 40 a anda determination circuit 52 a that determines the current sampled by thecurrent sampling circuit 51 a and that issues a lifting speedinstruction to the lifting motor 16.

The current sampling circuit 51 a monitors a current I_(power1) outputfrom the welding power source 30 a and flowing through the secondarycable 40 a.

When the current I_(power1) is smaller than the set value of currentI_(set1), the determination circuit 52 a issues an instruction todecrease a rising speed S_(up) of the carriage 15, and when the currentI_(power1) is equal to the set value of current I_(set1), thedetermination circuit 52 a issues an instruction to maintain the risingspeed S_(up) of the carriage 15. When the current I_(power1) is largerthan the set value of current I_(set1), the determination circuit 52 aissues an instruction to increase the rising speed S_(up).

The voltage adjustment circuit 60 a includes a voltage sampling circuit61 a that samples the voltage output from the welding power source 30 a,a waveform analysis circuit 62 a that analyzes the waveform of thevoltage sampled by the voltage sampling circuit 61 a and that outputs aresult of the analysis, and a determination circuit 63 a that issues anoutput voltage instruction to the welding power source 30 a inaccordance with the result of the analysis obtained by the waveformanalysis circuit 62 a.

The voltage sampling circuit 61 a monitors voltage V_(power1) outputfrom the welding power source 30 a during the welding.

The waveform analysis circuit 62 a counts a number of times N_(short1)[times/s] in unit time that the voltage V_(power1) falls below thecertain determination voltage V_(short1). Alternatively, the waveformanalysis circuit 62 a calculates an average short circuit periodAve_(Tshort1) [s] of a certain number of periods for which the voltageV_(power1) remains below the certain determination voltage V_(short1).

When the number of times N_(short1) is larger than the set number oftimes N_(set1), the determination circuit 63 a issues an instruction toincrease the voltage V_(power1), and when the number of times N_(short1)is equal to the set number of times N_(set1), the determination circuit63 a issues an instruction to maintain the voltage V_(power1). When thenumber of times N_(short1) is smaller than the set number of timesN_(set1), the determination circuit 63 a issues an instruction todecrease the voltage V_(power1). Alternatively, when the average shortcircuit period Ave_(Tshort1) of the certain number of periods is longerthan the set time T_(set1), the determination circuit 63 a issues aninstruction to increase the voltage V_(power1), and when the averageshort circuit period AVe_(Tshort1) is equal to the set time T_(set1),the determination circuit 63 a issues an instruction to maintain thevoltage V_(power1). When the average short circuit period Ave_(Tshort1)is shorter than the set time T_(set1), the determination circuit 63 aissues an instruction to decrease the voltage V_(power1).

The voltage adjustment circuit 60 b includes a voltage sampling circuit61 b that samples the voltage output from the welding power source 30 b,a waveform analysis circuit 62 b that analyzes the waveform of thevoltage sampled by the voltage sampling circuit 61 b and that outputs aresult of the analysis, and a determination circuit 63 b that issues anoutput voltage instruction to the welding power source 30 b inaccordance with the result of the analysis obtained by the waveformanalysis circuit 62 b.

The voltage sampling circuit 61 b monitors voltage V_(power2) outputfrom the welding power source 30 b during welding.

The waveform analysis circuit 62 b counts a number of times N_(short2)[times/s] in unit time that the voltage V_(power2) falls below thecertain determination voltage V_(short2). Alternatively, the waveformanalysis circuit 62 b calculates the average short circuit periodAve_(Tshort2) [s] of a certain number of periods for which the voltageV_(power2) remains below the certain determination voltage V_(short2).

When the number of times N_(short2) is larger than the set number oftimes N_(set2), the determination circuit 63 b issues an instruction toincrease the voltage V_(power2), and when the number of times N_(short2)is equal to the set number of times N_(set2), the determination circuit63 b issues an instruction to maintain the voltage V_(power2). When thenumber of times N_(short2) is smaller than the set number of timesN_(set2), the determination circuit 63 b issues an instruction todecrease the voltage V_(power2). Alternatively, when the average shortcircuit period Ave_(Tshort2) of the certain number of periods is longerthan the set time T_(set2), the determination circuit 63 b issues aninstruction to increase the voltage V_(power2), and when the averageshort circuit period Ave_(Tshort2) is equal to the set time T_(set2),the determination circuit 63 b issues an instruction to maintain thevoltage V_(power2). When the average short circuit period Ave_(Tshort2)is shorter than the set time T_(set2), the determination circuit 63 bissues an instruction to decrease the voltage V_(power2).

FIG. 6 is a schematic diagram of the configuration of a weldingapparatus 1 in the related art as a comparative example of the weldingapparatus 1 according to the second embodiment.

The welding apparatus 1 in the related art does not include the voltageadjustment circuits 60 a and 60 b, which are included in the weldingapparatus 1 according to the second embodiment illustrated in FIG. 5,and the welding conditions thereof specified by the operation displaybox 20 only include set values of voltage V_(set1) and V_(set2) and donot include the determination voltages V_(short1) and V_(short2), theset numbers of times N_(set1) and N_(set2), and the set times T_(set1)and T_(set2). Therefore, detailed description of each component isomitted.

Although the voltage adjustment circuit 60 a is configured as aso-called control box independent of the welding power source 30 a andthe voltage adjustment circuit 60 b is configured as a so-called controlbox independent of the welding power source 30 b in the secondembodiment, the present invention is not limited to this. When thevoltage adjustment circuit 60 a is included in the welding power source30 a and the voltage adjustment circuit 60 b is included in the weldingpower source 30 b and then the welding power sources 30 a and 30 b areused as dedicated power sources, portability and connectability improve,which makes the welding power sources 30 a and 30 b more convenient.

Although not illustrated in FIG. 5, a voltage adjustment function(realized by a dial or digital setting) that adjusts the output voltagesto the set values of voltage V_(set1) and V_(set2) and that isinvariably used by general welding power sources 30 a and 30 b is validonly during arc startup herein regardless of whether the voltageadjustment circuits 60 a and 60 b are control boxes or included in thewelding power sources 30 a and 30 b, respectively. This is because thevoltage adjustment function is generally a function of enabling anoperator to consciously adjust the arc length, but since the arc lengthis automatically adjusted to an optimal value in the present embodiment,an adjustment operation performed by the operator using his/her ownphysical functions and determinations is not necessary whereas, onlyduring the arc startup, the control according to the present embodimentdoes not function due to its calculation mechanism. That is, the voltageadjustment function according to the present embodiment exists todetermine arc startup voltage. However, because the voltage adjustmentfunction hardly affects the welding quality, the importance of thevoltage adjustment function is significantly lower than that in thewelding apparatus 1 in the related art.

FIG. 7 is a flowchart illustrating an example of the operation of thespeed adjustment circuit 50 illustrated in FIG. 3.

As illustrated in FIG. 7, first, the current sampling circuit 51 of thespeed adjustment circuit 50 samples the current I_(power) flowingthrough the secondary cables 40 (step 501).

Next, the determination circuit 52 compares the current I_(power) withthe set value of current I_(set) (step 502). As a result, if the currentI_(power) is smaller than the set value of current I_(set), thedetermination circuit 52 issues an instruction to decrease the risingspeed S_(up) of the carriage 15 to the lifting motor 16 (step 503). Onthe other hand, if the current I_(power) is larger than the set value ofcurrent I_(set), the determination circuit 52 issues an instruction toincrease the rising speed S_(up) of the carriage 15 to the lifting motor16 (step 504). If the current I_(power) is equal to the set value ofcurrent I_(set), the determination circuit 52 issues an instruction tomaintain the rising speed S_(up) of the carriage 15 to the lifting motor16 (step 505).

An example of the operation of the speed adjustment circuit 50 aillustrated in FIG. 5 is the same as the above-described example. Inthis case, however, the speed adjustment circuit 50, the currentsampling circuit 51, the determination circuit 52, the secondary cables40, the current I_(power), and the set value of current Let in theflowchart of FIG. 7 and the above description need to be replaced by thespeed adjustment circuit 50 a, the current sampling circuit 51 a, thedetermination circuit 52 a, the secondary cable 40 a, the currentI_(power1), and the set value of current I_(set1), respectively.

FIG. 8 is a flowchart illustrating a first example of the operation ofthe voltage adjustment circuit 60 illustrated in FIG. 3. In the firstexample of the operation, as illustrated in FIG. 1, the number of shortcircuits is used as the parameter for controlling the arc length.

As illustrated in FIG. 8, first, the voltage sampling circuit 61 of thevoltage adjustment circuit 60 samples the voltage V_(power) output fromthe welding power source 30 (step 601).

Next, the waveform analysis circuit 62 analyzes the waveform of thevoltage V_(power), and calculates the number of times N_(short)[times/s] in unit time that the voltage V_(power) falls below thedetermination voltage V_(short) (step 602).

Next, the determination circuit 63 compares the number of timesN_(short) with the set number of times N_(set) (step 603). As a result,if the number of times N_(short) is larger than the set number of timesN_(set), the determination circuit 63 issues an instruction to increasethe voltage V_(power) to the welding power source 30 (step 604). On theother hand, if the number of times N_(short) is smaller than the setnumber of times N_(set), the determination circuit 63 issues aninstruction to decrease the voltage V_(power) to the welding powersource 30 (step 605). If the number of times N_(short) is equal to theset number of times N_(set), the determination circuit 63 issues aninstruction to maintain the voltage V_(power) to the welding powersource 30 (step 606).

An example of the operation of the voltage adjustment circuit 60 aillustrated in FIG. 5 is the same as the above-described example whenthe number of short circuits is used as the parameter for controllingthe arc length. In this case, however, the voltage adjustment circuit60, the voltage sampling circuit 61, the waveform analysis circuit 62,the determination circuit 63, the welding power source 30, the voltageV_(power), the determination voltage V_(short), the number of timesN_(short), and the set number of times N_(set) in the flowchart of FIG.8 and the above description need to be replaced by the voltageadjustment circuit 60 a, the voltage sampling circuit 61 a, the waveformanalysis circuit 62 a, the determination circuit 63 a, the welding powersource 30 a, the voltage V_(power1), the determination voltageV_(short), the number of times N_(short), and the set number of timesN_(set1), respectively.

An example of the operation of the voltage adjustment circuit 60 billustrated in FIG. 5 is also the same as the above-described examplewhen the number of short circuits is used as the parameter forcontrolling the arc length. In this case, however, the voltageadjustment circuit 60, the voltage sampling circuit 61, the waveformanalysis circuit 62, the determination circuit 63, the welding powersource 30, the voltage V_(power), the determination voltage V_(short),the number of times N_(short), and the set number of times N_(set) inthe flowchart of FIG. 8 and the above description need to be replaced bythe voltage adjustment circuit 60 b, the voltage sampling circuit 61 b,the waveform analysis circuit 62 b, the determination circuit 63 b, thewelding power source 30 b, the voltage V_(power2), the determinationvoltage V_(short2), the number of times N_(short2), and the set numberof times N_(set2), respectively.

FIG. 9 is a flowchart illustrating a second example of the operation ofthe voltage adjustment circuit 60 illustrated in FIG. 3. In the secondexample of the operation, as illustrated in FIG. 2, the short circuitperiods are used as the parameter for controlling the arc length.

As illustrated in FIG. 9, first, the voltage sampling circuit 61 of thevoltage adjustment circuit 60 samples the voltage V_(power) output fromthe welding power source 30 (step 651).

Next, the waveform analysis circuit 62 analyzes the waveform of thevoltage V_(power), and calculates the average short circuit periodAve_(Tshort) [s] of the certain number of periods for which the voltageV_(power) remains below the determination voltage V_(short) (step 652).

Next, the determination circuit 63 compares the average short circuitperiod Ave_(Tshort) with the set time T_(set) (step 653). As a result,if the average short circuit period Ave_(Tshort) is longer than the settime T_(set), the determination circuit 63 issues an instruction toincrease the voltage V_(power) to the welding power source 30 (step654). On the other hand, if the average short circuit periodAve_(Tshort) is shorter than the set time T_(set), the determinationcircuit 63 issues an instruction to decrease the voltage V_(power) tothe welding power source 30 (step 655). If the average short circuitperiod Ave_(Tshort) is equal to the set time T_(set), the determinationcircuit 63 issues an instruction to maintain the voltage V_(power) tothe welding power source 30 (step 656).

An example of the operation of the voltage adjustment circuit 60 aillustrated in FIG. 5 is the same as the above-described example whenthe short circuit periods are used as the parameter for controlling thearc length. In this case, however, the voltage adjustment circuit 60,the voltage sampling circuit 61, the waveform analysis circuit 62, thedetermination circuit 63, the welding power source 30, the voltageV_(power), the determination voltage V_(short), the average shortcircuit period Ave_(Tshort), and the set time T_(set) in the flowchartof FIG. 9 and the above description need to be replaced by the voltageadjustment circuit 60 a, the voltage sampling circuit 61 a, the waveformanalysis circuit 62 a, the determination circuit 63 a, the welding powersource 30 a, the voltage V_(power1), the determination voltageV_(short1), the average short circuit period Ave_(Tshort), and the settime T_(set1), respectively.

An example of the operation of the voltage adjustment circuit 60 billustrated in FIG. 5 is also the same as the above-described examplewhen the short circuit periods are used as the parameter for controllingthe arc length. In this case, however, the voltage adjustment circuit60, the voltage sampling circuit 61, the waveform analysis circuit 62,the determination circuit 63, the welding power source 30, the voltageV_(power), the determination voltage V_(short), the average shortcircuit period Ave_(Tshort), and the set time T_(set) in the flowchartof FIG. 9 and the above description need to be replaced by the voltageadjustment circuit 60 b, the voltage sampling circuit 61 b, the waveformanalysis circuit 62 b, the determination circuit 63 b, the welding powersource 30 b, the voltage V_(power2), the determination voltageV_(short2), the average short circuit period Ave_(Tshort2), and the settime T_(set2), respectively.

Although the present embodiments are examples in which the presentinvention is applied to butt joint welding of plates in a verticalattitude, the present invention may be applied to any type ofvertical-up welding.

FIGS. 10A to 10F illustrate examples of welding to which the presentinvention may be applied.

That is, in addition to the butt joint welding of plates in the verticalattitude illustrated in FIG. 10A, the present invention may be appliedto corner welding illustrated in FIG. 10B, fillet welding of a T jointillustrated in FIG. 10C, groove welding of a T joint illustrated in FIG.10D, groove welding of a cylindrical pipe and a flange illustrated inFIG. 10E, circumferential butt welding of two cylindrical pipesillustrated in FIG. 10F, and the like. In FIGS. 10A to 10D, vertical-upwelding is realized by performing welding in directions indicated byillustrated hollow arrows. On the other hand, in FIGS. 10E and 10F,relative vertical welding is realized by performing welding on basemetals rotating in directions indicated by illustrated hollow arrowswhile fixing welding torches at a 3 o'clock position. In the relativevertical welding, however, the control of the rising speed realized by alifting motor using values of current is replaced by control of therotation speed of a base metal using values of current.

Although the inventors have tested the mechanisms according to thepresent embodiments in downhand welding and horizontal welding, whichare two of the most general types of welding, the mechanisms have notproduced advantageous effects, which has indicated that the mechanismsaccording to the present embodiments are particularly effective invertical-up welding. This is probably because of the following reason.That is, in vertical-up welding, because the molten pool P is completelysurrounded by the groove G, the backing material 13, and the slidingcopper plate 14, which means that no escape is possible, and the risingspeed is low, a distance between the surface of the molten pool P andthe welding wire may be stably maintained, and accordingly the controlis likely to become more and more accurate over time, thereby making themechanisms effective. On the other hand, in downhand welding andhorizontal welding, as can be seen from FIG. 11A, since there is noobject that physically blocks the molten pool in the moving direction ofthe arc, the molten pool is likely to irregularly flow forward due togravity and accordingly a distance between the welding wire and thesurface of the molten pool immediately below the welding wirecontinuously varies, which affects the control such that the controlbecomes inaccurate, thereby causing a large variation in the arc length.That is, the mechanisms according to the present embodiments presupposethat the position of the surface of the molten pool can be stablymaintained. Therefore, in the present embodiments, it is desirable tocontrol not only the voltage but also the rising speed through samplingof the current.

In addition, gas-shielded arc welding, which is one of the most generaltypes of welding, is a type of welding to which the present embodimentsmay be applied. In the gas-shielded arc welding, a solid wire or aflux-cored wire is used as the welding wire, and carbonic acid gas or amixed gas of argon and carbonic acid gas is blown onto the surface ofthe molten pool P in order to separate the molten pool P from theatmosphere and secure soundness. Alternatively, the present embodimentsmay be applied to non-gas-shielded arc welding. The non-gas-shielded arcwelding is also called self-shielded arc welding, in which a dedicatedflux-cored wire is used as the welding wire to perform welding withoutusing shielding gas. Although there are some disadvantages in that theamount of fumes generated is large and the toughness of the weld metalis low compared to that in the gas-shielded arc welding, thenon-gas-shielded arc welding requires no maintenance of a gas feedsystem and a gas supply port and equipment such as a gas cylinder or agas tank is not necessary, which are advantageous. The non-gas-shieldedarc welding is more preferable than the gas-shielded arc welding in awindy environment.

Thus, the following advantageous effects may be produced by the presentembodiments.

That is, in the vertical welding apparatuses in the related art, forexample, when the configuration of equipment has been changed inaccordance with an installation environment or when a failure in thesetting of the welding conditions has occurred unconsciously or due tothe skill of the operator, adverse effects on the welding quality areinevitable, and the operator needs to adjust the configuration by trialand error. Therefore, the operator needs to stay at the welding siteover a long period of time. However, in the vertical welding apparatusesaccording to the present embodiments, the arc length, which maysignificantly affect the welding quality, may be automatically adjustedto an optimal value. Therefore, thanks to the automatic adjustmentfunction, the operator may leave the welding site after generating anarc and starting the apparatus, thereby improving the quality andreducing costs, which is significantly advantageous.

Now, the limitation of values such as the parameters used in the presentembodiments and the reasons for the limitation will be described.

First, the set number of times N_(set) will be described.

A small set number of times N_(set) means that the control is to beperformed such that the number of short circuits becomes small, that is,the arc length becomes large. When the arc length is large, thepenetration becomes deep. However, when the arc length is too large,phenomena such as excessive oxidization of alloy elements of the weldmetal and mixing of the atmosphere in the arc may occur, whichdeteriorates the properties of the weld metal. If V_(short)=15 V, thesephenomena are likely to occur when the set number of times N_(set) issmaller than 3.

On the other hand, a large set number of times N_(set) means that thecontrol is performed such that the number of short circuits becomeslarge, that is, the arc length becomes small. When the arc length issmall, the mechanical properties of the weld metal improve. However,when the arc length is too small, the convection in the molten pool Pbecomes weak, which causes failures in the penetration. If V_(short)=15V, this phenomenon is likely to occur when the set number of timesN_(set) is larger than 60.

Therefore, the set number of times N_(set) is desirably 3 to 60 timesper second. By strictly setting the set number of times N_(set) to 5 to20 times per second, the properties of the weld metal and the depth ofpenetration are more balanced.

The same holds true for the set numbers of times N_(set1) and N_(set2).

Next, the set time T_(set) will be described.

A short set time T_(set) means that short circuits last only shortperiods of time (only small short circuits occur), that is, the arclength becomes large. When the arc length is large, the penetrationbecomes large. However, when the arc length is too large, phenomena suchas excessive oxidization of the alloy elements of the weld metal andmixing of the atmosphere in the arc may occur, which deteriorates theproperties of the weld metal. If V_(short)=15 V, these phenomena arelikely to occur when the set time T_(set) is shorter than 0.1 ms.

On the other hand, a long set time T_(set) means that short circuitslast long periods of time (the magnitude of short circuits is large),that is, the arc length becomes small. When the arc length is small, themechanical properties of the weld metal improve. However, when the arclength is too small, the convection in the molten pool P becomes weak,which causes failures in the penetration. If V_(short)=15 V, thisphenomenon is likely to occur when the set time T_(set) is longer than1.0 ms.

Therefore, the set time T_(set) is desirably 0.1 to 1.0 ms. By strictlysetting the set time T_(set) to 0.2 to 0.5 ms, the properties of theweld metal and the depth of penetration are more balanced.

The same holds true for the set times T_(set1) and T_(set2).

Next, the rising speed S_(up) determined by the lifting motor 16 will bedescribed.

In the present embodiments, since a routine including sampling ofvoltage, an analysis, a determination, and an output voltage instructionto the welding power source 30 (30 a and 30 b) and a routine includingsampling of current, a determination, a lifting speed instruction to thelifting motor 16 are simultaneously repeated in order to stabilize thearc length, the control becomes more effective as the surface of themolten pool P becomes smoother and more stable. Since the risingdirections of the welder and the molten pool P and the direction of thearc length are the same, the decrease in the arc length in routineperiods becomes larger as the rising speed S_(up) becomes higher.Therefore, the degree of correction realized as a result of the controlaccording to the present embodiments becomes high and rough, therebymaking it difficult for the arc length to be appropriate.

Because of the above-described mechanism, the control for stabilizingthe arc length becomes more desirable when the rising speed S_(up) issmaller. More specifically, the control according to the presentembodiments becomes effective when the rising speed S_(up) is lower thanor equal to 180 mm/min. By setting the rising speed S_(up) to be lowerthan or equal to 120 mm/min, the arc length may be maintained moreproperly. Therefore, it is desirable to set the rising speed S_(up) tobe lower than or equal to 120 mm/min. Such rising speed S_(up) issufficiently practical in vertical welding.

EXAMPLES

Next, examples of the present invention will be described whilecomparing the examples with comparative examples that fall out of thescope of the present invention. The examples and the comparativeexamples provide reasons to use the above-described limitation ofvalues.

First Examples One-Electrode Type

Vertical-up welding was performed using JIS G3106 SM490B carbon steelplates having a thickness of 12 mm as steel plates while performinggroove processing such that a 50° V-groove and a root gap of 5 mm wereobtained. Gas-shielded arc welding was applied, a JIS Z3319 YFEG-22Cflux-cored wire having a diameter of 1.6 mm was used as the weldingwire, and CO₂ was used as the shielding gas. The welding apparatus usedthe configuration illustrated in FIG. 3 or 4. Welding of a weldinglength of 500 mm was performed while changing the welding conditions andthe determination conditions, and, after the welding, an ultrasonic flawdetection test for evaluating the penetration and a Charpy impact testof the center of a cross-section, which is one of methods for evaluatingthe properties of a weld metal, were performed. In the ultrasonic flawdetection test, a double circle was entered when there had been nofailure in the penetration, a circle was entered when there had been oneto three failures in the penetration, and an X mark was entered whenthere had been four or more failures in the penetration. In the Charpyimpact test, under a test temperature of −20° C., a double circle wasentered when a Charpy impact value had been equal to or larger than 47J, a circle was entered when the Charpy impact value had been equal toor larger than 27 J but smaller than 47 J, and an X mark was enteredwhen the Charpy impact value had been smaller than 27 J.

Results of the tests are indicated in the table below. In the table,conditions different from the base conditions are indicated in bolditalic characters, and results of the ultrasonic flaw detection testsand the Charpy impact tests that pose problems are indicated in boldcharacters surrounded by bold frames.

TABLE 1

No. A1 is a case in which the arc length was controlled on the basis ofthe number of short circuits using the configuration illustrated in FIG.3. No. A2 is a case in which the arc length was controlled on the basisof the average short circuit period using the configuration illustratedin FIG. 3. No. A3 is a case in which the configuration illustrated inFIG. 4, which does not include a voltage adjustment circuit forcontrolling the arc length, was used. In No. A1 to No. A3, setting wasmade such that the arc length became optimal when the length of thesecondary cables was 25 m. Results of the ultrasonic flaw detectiontests and the Charpy impact tests performed in No. A1 to No. A3 posed noproblems.

No. A4 to No. A6 are cases in which the length of the secondary cableswas extended to 50 m while using the above-described setting as areference. In No. A4 and No. A5, in which the configuration illustratedin FIG. 3 was used, the arc length remained optimal since the arc lengthwas automatically controlled even under the welding conditions at a timewhen the length of the secondary cables is 25 m. Accordingly, results ofthe ultrasonic flaw detection tests and the Charpy impact tests posed noproblems. On the other hand, in No. A6, in which the configurationillustrated in FIG. 4 was used, the arc voltage became relatively lowsince the secondary cables became longer and the voltage loss in thesecondary cables became larger. Accordingly, the convection in themolten pool became weaker and the penetration became shallower, therebycausing a lot of failures in the ultrasonic flaw detection test.

In contrast, No. A7 to No. A9 are cases in which the length of thesecondary cables was reduced to 10 m. In No. A7 and No. A8, in which theconfiguration illustrated in FIG. 3 was used, the arc length remainedoptimal since the arc length was automatically controlled even under thewelding conditions at a time when the length of the secondary cables is25 m. Accordingly, results of the ultrasonic flaw detection tests andthe Charpy impact tests posed no problems. On the other hand, in No. A9,in which the configuration illustrated in FIG. 4 was used, the arcvoltage became relatively high since the secondary cables became shorterand the voltage loss in the secondary cables became smaller.Accordingly, the arc length became too large, and phenomena such asexcessive oxidization of the alloy elements of the weld metal and mixingof the atmosphere in the arc occurred, which deteriorated theperformance of the weld metal in the Charpy impact test.

No. A10 to No. A12 are cases in which voltage was set lower than that inNo. A₁ to No. A3 by mistake. In No. A10 and No. A11, in which theconfiguration illustrated in FIG. 3 was used, the arc length wasmomentarily unstable during the arc startup, but thereafter the weldingwas performed using an optimal arc length since the voltage setting wasused only as an arc startup condition and the arc length was controlledimmediately. Therefore, the soundness of the penetration and the weldmetal was maintained. On the other hand, in No. A12, in which theconfiguration illustrated in FIG. 4 was used, because the welding wasperformed under the low voltage condition set by mistake, the convectionin the molten pool became weaker and the penetration became shallower,thereby causing a lot of failures in the ultrasonic flaw detection test.

No. A13 to No. A15 are cases in which voltage was set higher than thatin No. A1 to A3 by mistake. In No. A13 and No. A14, in which theconfiguration illustrated in FIG. 3 was used, the arc length wasmomentarily unstable during the arc startup, but thereafter the weldingwas performed using an optimal arc length since the voltage setting wasused only as an arc startup condition and the arc length was controlledimmediately. Therefore, the soundness of the penetration and the weldmetal was maintained. On the other hand, in No. A15, in which theconfiguration illustrated in FIG. 4 was used, because the welding wasperformed under the high voltage condition set by mistake, the arclength became too large, and phenomena such as excessive oxidization ofthe alloy elements of the weld metal and mixing of the atmosphere in thearc occurred, which deteriorated the performance of the weld metal inthe Charpy impact test.

No. A16 to No. A19 are cases in which the set number of times N_(set),which is a threshold for the number of short circuits, was differentfrom that in No. A1. In No. A16 and No. A18, in which the set number oftimes N_(set) was within the most desirable range, the penetrationperformance and the properties of the weld metal were satisfactory. InNo. A17, however, in which the set number of times net was set low, thearc length became slightly too large, and the Charpy impact test waspassed but the Charpy impact value was relatively small. On the otherhand, in No. A19, in which the set number of times N_(set) was set high,the arc length became slightly too small, and the ultrasonic flowdetection test was passed but a small number of failures occurred.

No. A20 is a case in which an upper limit value of the rising speedallowed in the welding using the configuration illustrated in FIG. 3 wasused. The control of the arc length was effective, and results of theultrasonic flow detection test and the Charpy impact test wererelatively satisfactory. In No. A21, however, in which a value of risingspeed exceeding the range allowed in the welding using the configurationillustrated in FIG. 3 was used, the control of the arc length was notable to come up with the rising speed of the surface of the molten pool,and accordingly the arc length and the penetration became unstable,thereby failing the ultrasonic flow detection test.

Second Examples One-Electrode Type

As with the first examples, vertical-up welding was performed using JISG3106 SM490B carbon steel plates having a thickness of 12 mm as steelplates while performing groove processing such that a 50° V-groove and aroot gap of 5 mm were obtained. Non-gas-shielded arc welding wasapplied, and a JIS Z3313 T49YT4-0NA wire having a diameter of 2.4 mm wasused as the welding wire. The welding apparatus used the configurationillustrated in FIG. 3 or 4. Welding of a welding length of 500 mm wasperformed while changing the welding conditions and the determinationconditions, and, after the welding, the ultrasonic flaw detection testfor evaluating the penetration and the Charpy impact test of the centerof a cross-section, which is one of methods for evaluating a weld metal,were performed. In the ultrasonic flaw detection test, a double circlewas entered when there had been no failure in the penetration, a circlewas entered when there had been one to three failures in thepenetration, and an X mark was entered when there had been four or morefailures in the penetration. In the Charpy impact test, under a testtemperature of +20° C., a double circle was entered when the Charpyimpact value had been equal to or larger than 47 J, a circle was enteredwhen the Charpy impact value had been equal to or larger than 27 J butsmaller than 47 J, and an X mark was entered when the Charpy impactvalue had been smaller than 27 J.

Results of the tests are indicated in the table below. In the table,conditions different from the base conditions are indicated in bolditalic characters, and results of the ultrasonic flaw detection testsand the Charpy impact tests that pose problems are indicated in boldcharacters surrounded by bold frames.

TABLE 2

No. B1 is a case in which the arc length was controlled on the basis ofthe number of short circuits using the configuration illustrated in FIG.3. No. B2 is a case in which the arc length was controlled on the basisof the average short circuit period using the configuration illustratedin FIG. 3. No. B3 is a case in which the configuration illustrated inFIG. 4, which does not include a voltage adjustment circuit forcontrolling the arc length, was used. In No. B1 to No. B3, setting wasmade such that the arc length became optimal when the length of thesecondary cables was 25 m. Results of the ultrasonic flaw detectiontests and the Charpy impact tests performed in No. B1 to No. B3 posed noproblems.

No. B4 to No. B6 are cases in which the length of the secondary cableswas extended to 50 m while using the above-described setting as areference. In No. B4 and No. B5, in which the configuration illustratedin FIG. 3 was used, the arc length remained optimal since the arc lengthwas automatically controlled even under the welding conditions at a timewhen the length of the secondary cables is 25 m. Accordingly, results ofthe ultrasonic flaw detection tests and the Charpy impact tests posed noproblems. On the other hand, in No. B6, in which the configurationillustrated in FIG. 4 was used, the arc voltage became relatively lowsince the secondary cables became longer and the voltage loss in thesecondary cables became larger. Accordingly, the convection in themolten pool became weaker and the penetration became shallower, therebycausing a lot of failures in the ultrasonic flaw detection test.

No. B7 to No. B10 are cases in which the set time T_(set), which is athreshold for the average short circuit period, was different from thatin No. B2. In No. B7 and No. B9, in which the set time T_(set) waswithin the most desirable range, the penetration performance and theproperties of the weld metal were satisfactory. In No. B8, however, inwhich the set time T_(set) was set short, the arc length became slightlytoo large, and the Charpy impact test was passed but the Charpy impactvalue was relatively small. On the other hand, in No. B10, in which theset time T_(set) was set long, the arc length became slightly too small,and the ultrasonic flow detection test was passed but a small number offailures occurred.

The conditions in No. B11 were similar to those in No. B1, but thepolarity of current was electrode negative, that is, so-called straightpolarity. The rising speed was also different since the wire meltingrate changes when the polarity has changed, but the control of the arclength according to the embodiments was effective and posed no problem.The penetration performance and the properties of the weld metal thathad been obtained posed no problems, either.

Third Examples Two-Electrode Type

Vertical-up welding was performed using JIS G3106 SM490C carbon steelplates having a thickness of 80 mm as steel plates while performinggroove processing such that a 20° V-groove and a root gap of 8 mm wereobtained. Gas-shielded arc welding was applied, two JIS Z3319 YFEG-22Cflux-cored wires having a diameter of 1.6 mm were used as the weldingwires, and CO₂ was used as the shielding gas. The welding apparatus usedthe configuration illustrated in FIG. 5 or 6, in which two electrodearcs form one molten pool and simultaneously rise. Welding of a weldinglength of 500 mm was performed while changing the welding conditions andthe determination conditions, and, after the welding, the ultrasonicflaw detection test for evaluating the penetration and the Charpy impacttest of the center of a cross-section, which is one of methods forevaluating a weld metal, were performed. In the ultrasonic flawdetection test, a double circle was entered when there had been nofailure in the penetration, a circle was entered when there had been oneto three failures in the penetration, and an X mark was entered whenthere had been four or more failures in the penetration. In the Charpyimpact test, under a test temperature of −20° C., a double circle wasentered when the Charpy impact value had been equal to or larger than 47J, a circle was entered when the Charpy impact value had been equal toor larger than 27 J but smaller than 47 J, and an X mark was enteredwhen the Charpy impact value had been smaller than 27 J.

Results of the tests are indicated in the table below. In the table,conditions different from the base conditions are indicated in bolditalic characters, and results of the ultrasonic flaw detection testsand the Charpy impact tests that pose problems are indicated in boldcharacters surrounded by bold frames.

TABLE 3

No. C1 is a case in which the two arc lengths were independentlycontrolled on the basis of the numbers of short circuits using theconfiguration illustrated in FIG. 5. No. C2 is a case in which the twoarc lengths were independently controlled on the basis of the averageshort circuit periods using the configuration illustrated in FIG. 5. No.C3 is a case in which the configuration illustrated in FIG. 6, whichdoes not include voltage adjustment circuits for controlling the two arclengths, was used. The rising speed of the welder was controlled byvalues of current sampled by the power supply system connected to thecloser electrode. In No. C1 to C3, setting was made such that the arclength became optimal when the length of the secondary cables was 25 m.Results of the ultrasonic flaw detection tests and the Charpy impacttests performed in No. C1 to No. C3 posed no problems.

No. C4 to No. C6 are cases in which the length of the secondary cableswas extended to 50 m while using the above-described setting as areference. In No. C4 and No. C5, in which the configuration illustratedin FIG. 5 was used, the arc length remained optimal since the arc lengthwas automatically controlled even under the welding conditions at a timewhen the length of the secondary cables is 25 m. Accordingly, results ofthe ultrasonic flaw detection tests and the Charpy impact tests posed noproblems. On the other hand, in No. C6, in which the configurationillustrated in FIG. 6 was used, the arc voltage became relatively lowsince the secondary cables became longer and the voltage loss in thesecondary cables became larger. Accordingly, the convection in themolten pool became weaker and the penetration became smaller, therebycausing a lot of failures in the ultrasonic flaw detection test.

In contrast, No. C7 to No. C9 are cases in which the length of thesecondary cables was reduced to 10 m. In No. C7 and No. C8, in which theconfiguration illustrated in FIG. 5 was used, the arc length remainedoptimal since the arc length was automatically controlled even under thewelding conditions at a time when the length of the secondary cables is25 m. Accordingly, results of the ultrasonic flaw detection tests andthe Charpy impact tests posed no problems. On the other hand, in No. C9,in which the configuration illustrated in FIG. 6 was used, the arcvoltage became relatively high since the secondary cables became shorterand the voltage loss in the secondary cables became smaller.Accordingly, the arc length became too large, and phenomena such asexcessive oxidization of the alloy elements of the weld metal and mixingof the atmosphere in the arc occurred, which deteriorated theperformance of the weld metal in the Charpy impact test.

Fourth Examples Two-Electrode Type

As with the third examples, vertical-up welding was performed using JISG3106 SM490C carbon steel plates having a thickness of 80 mm as steelplates while performing groove processing such that a 20° V-groove and aroot gap of 8 mm were obtained. Gas-shielded arc welding was applied,two JIS Z3319 YFEG-22C flux-cored wires having a diameter of 1.6 mm wereused as the welding wires, and CO₂ was used as the shielding gas. Thewelding apparatus used the configuration illustrated in FIG. 5 or 6, inwhich two electrode arcs form one molten pool and simultaneously rise.Welding of a welding length of 500 mm was performed while changing thewelding conditions and the determination conditions, and, after thewelding, the ultrasonic flaw detection test for evaluating thepenetration and the Charpy impact test of the center of a cross-section,which is one of methods for evaluating a weld metal, were performed. Inthe ultrasonic flaw detection test, a double circle was entered whenthere had been no failure in the penetration, a circle was entered whenthere had been one to three failures in the penetration, and an X markwas entered when there had been four or more failures in thepenetration. In the Charpy impact test, under a test temperature of −20°C., a double circle was entered when the Charpy impact value had beenequal to or larger than 47 J, a circle was entered when the Charpyimpact value had been equal to or larger than 27 J but smaller than 47J, and an X mark was entered when the Charpy impact value had beensmaller than 27 J.

Results of the tests are indicated in the table below. In the table,conditions different from the base conditions are indicated in bolditalic characters, and results of the ultrasonic flaw detection testsand the Charpy impact tests that pose problems are indicated in boldcharacters surrounded by bold frames.

TABLE 4

No. D1 is a case in which the two arc lengths were independentlycontrolled on the basis of the average short circuit periods using theconfiguration illustrated in FIG. 5. No. D2 is a case in which theconfiguration illustrated in FIG. 6, which does not include voltageadjustment circuits for controlling the two arc lengths, was used. Therising speed of the welder was controlled by values of current sampledby the power supply system connected to the closer electrode. In No. D1and D2, setting was made such that the arc length became optimal whenthe length of the secondary cables was 25 m although the combinationbetween electrode polarities was different from that in No. C2 and No.C3. Accordingly, results of the ultrasonic flaw detection tests and theCharpy impact tests performed in No. D1 and No. D2 posed no problems.

No. D3 and No. D4 are cases in which voltage was set lower than that inNo. D1 and No. D2 by mistake. In No. D3, in which the configurationillustrated in FIG. 5 was used, the arc length was momentarily unstableduring the arc startup, but thereafter the welding was performed usingan optimal arc length since the voltage setting was used only as an arcstartup condition and the arc length was controlled immediately.Therefore, the soundness of the penetration and the weld metal wasmaintained. On the other hand, in No. D4, in which the configurationillustrated in FIG. 6 was used, because the welding was performed underthe low voltage condition set by mistake, the convection in the moltenpool became weaker and the penetration became shallower, thereby causinga lot of failures in the ultrasonic flaw detection test.

No. D5 and No. D6 are cases in which voltage was set higher than that inNo. D1 and D2 by mistake. In No. D5, in which the configurationillustrated in FIG. 5 was used, the arc length was momentarily unstableduring the arc startup, but thereafter the welding was performed usingan optimal arc length since the voltage setting was used only as an arcstartup condition and the arc length was controlled immediately.Therefore, the soundness of the penetration and the weld metal wasmaintained. On the other hand, in No. D6, in which the configurationillustrated in FIG. 6 was used, because the welding was performed underthe high voltage condition set by mistake, the arc length became toolarge and phenomena such as excessive oxidization of the alloy elementsof the weld metal and mixing of the atmosphere in the arc occurred,which deteriorated the performance of the weld metal in the Charpyimpact test.

What is claimed is:
 1. An arc welding apparatus that performsvertical-up welding by generating an arc in a groove between steelplates to be welded and forming a molten pool, the arc welding apparatuscomprising: welding means for performing arc welding by generating thearc from a welding wire in the groove between the steel plates to bewelded in a substantially vertically downward direction and forming themolten pool; lifting means for lifting the welding means in asubstantially vertically upward direction relative to the steel platesto be welded; a welding power source that feeds current to the weldingwire to generate the arc; speed control means for monitoring currentoutput from the welding power source and, if a value of the outputcurrent is smaller than a value of current set in advance, controllingthe lifting means such that rising speed of the welding means decreasesor, if the value of the output current is larger than the set value ofcurrent, controlling the lifting means such that the rising speed of thewelding means increases; and voltage control means for monitoringvoltage output from the welding power source during the welding,obtaining information regarding the number of times that a value of theoutput voltage falls below a determination voltage, which is set inadvance as a determination threshold, or periods for which the value ofthe output voltage remains below the determination voltage and, if theinformation regarding the number of times or the periods exceeds a setthreshold, controlling the welding power source such that the value ofthe output voltage increases or, if the information regarding the numberof times or the periods is below the set threshold, controlling thewelding power source such that the value of the output voltagedecreases.
 2. The arc welding apparatus according to claim 1, whereinthe information regarding the number of times that the value of theoutput voltage falls below the determination voltage or the periods forwhich the value of the output voltage remains below the determinationvoltage is the number of times in unit time that the value of the outputvoltage falls below the determination voltage, and wherein the setthreshold is the number of times set in advance as the number of timesin the unit time.
 3. The arc welding apparatus according to claim 2,wherein, when the determination voltage is 15 V, the set number of timesis any number of times within a range from 3 times per second to 60times per second.
 4. The arc welding apparatus according to claim 1,wherein the information regarding the number of times that the value ofthe output voltage falls below the determination voltage or the periodsfor which the value of the output voltage remains below thedetermination voltage is a period obtained on the basis of apredetermined number of periods for which the value of the outputvoltage remains below the determination voltage, and wherein the setthreshold is a time set in advance.
 5. The arc welding apparatusaccording to claim 4, wherein, when the determination voltage is 15 V,the set time is any time within a range from 0.1 ms to 1.0 ms.
 6. Thearc welding apparatus according to claim 3, wherein the speed controlmeans controls the lifting means such that the rising speed of thewelding means becomes lower than or equal to 180 mm/min.
 7. An arcwelding apparatus that performs vertical-up welding by generating an arcin a groove between steel plates to be welded and forming a molten pool,the arc welding apparatus comprising: a backing material mounted acrossa root gap provided in a back of the groove between the steel plates tobe welded; welding means for performing arc welding by generating thearc from a welding wire in the groove between the steel plates to bewelded in a substantially vertically downward direction and forming themolten pool, the welding means including a welding torch that isarranged in front of the groove between the steel plates to be weldedand that supplies the welding wire into the groove, a weaving mechanismthat oscillates the welding torch in a width direction of the groove,and a sliding copper plate that relatively slides over front surfaces ofthe steel plates to be welded in a substantially vertically upwarddirection; lifting means for lifting the welding means in thesubstantially vertically upward direction relative to the steel platesto be welded; a welding power source that feeds current to the weldingwire to generate the arc; speed control means for monitoring currentoutput from the welding power source and, if a value of the outputcurrent is smaller than a value of current set in advance, controllingthe lifting means such that rising speed of the welding means decreasesor, if the value of the output current is larger than the set value ofcurrent, controlling the lifting means such that the rising speed of thewelding means increases; and voltage control means for monitoringvoltage output from the welding power source during the welding,obtaining information regarding the number of times that a value of theoutput voltage falls below a determination voltage, which is set inadvance as a determination threshold, or periods for which the value ofthe output voltage remains below the determination voltage and, if theinformation regarding the number of times or the periods exceeds a setthreshold, controlling the welding power source such that the value ofthe output voltage increases or, if the information regarding the numberof times or the periods is below the set threshold, controlling thewelding power source such that the value of the output voltagedecreases.
 8. A constant voltage characteristic welding power sourceused for an arc welding apparatus that performs vertical-up welding bylifting, in a substantially vertically upward direction relative tosteel plates to be welded, a welder that performs welding by generatingan arc from a welding wire in a groove between the steel plates to bewelded in a substantially vertically downward direction and forming amolten pool, the constant voltage characteristic welding power sourcecomprising: power supply means for feeding current to the welding wireto generate the arc; speed control means for monitoring current outputfrom the power supply means and, if a value of the output current issmaller than a value of current set in advance, performing control suchthat rising speed of the welder decreases or, if the value of the outputcurrent is larger than the set value of current, performing control suchthat the rising speed of the welder increases; and voltage control meansfor monitoring voltage output from the power supply means during thewelding, obtaining information regarding the number of times that avalue of the output voltage falls below a determination voltage, whichis set in advance as a determination threshold, or periods for which thevalue of the output voltage remains below the determination voltage and,if the information regarding the number of times or the periods exceedsa set threshold, performing control such that the value of the outputvoltage increases or, if the information regarding the number of timesor the periods is below the set threshold, performing control such thatthe value of the output voltage decreases.
 9. A method for performingarc welding in which vertical-up welding is performed by lifting, in asubstantially vertically upward direction relative to steel plates to bewelded, a welder that performs welding by generating an arc from awelding wire in a groove between the steel plates to be welded in asubstantially vertically downward direction and forming a molten pool,the method comprising the steps of: monitoring current output from awelding power source that feeds current to the welding wire to generatethe arc and, if a value of the output current is smaller than a value ofcurrent set in advance, performing control such that rising speed of thewelder decreases or, if the value of the output current is larger thanthe set value of current, performing control such that the rising speedof the welder increases; and monitoring voltage output from the weldingpower source during the welding, obtaining information regarding thenumber of times that a value of the output voltage falls below adetermination voltage, which is set in advance as a determinationthreshold, or periods for which the value of the output voltage remainsbelow the determination voltage and, if the information regarding thenumber of times or the periods exceeds a set threshold, performingcontrol such that the value of the output voltage increases or, if theinformation regarding the number of times or the periods is below theset threshold, performing control such that the value of the outputvoltage decreases.